The invention relates to flexible multilayer laminates comprising at least one layer of a no longer formable, fully aromatic polyimide and at least one layer of a substrate material. The invention also relates to a process for producing these laminates.
Laminates comprising one or more layers of polyimide and one or more layers of substrate material may be used for a variety of applications, such as, for example, reinforcing materials. In addition, laminates of the type in question, in the form of polyimide-coated metal foils, are used for printed electrical circuits. In that case, use is made of the flexibility and outstanding mechanical, thermal and electrical properties of the polyimides. This is because the laminates are frequently exposed to high temperatures during further processing, for example during soldering or drilling. The laminates also have to satisfy stringent requirements regarding their electrical and mechanical properties.
Laminates comprising only one substrate layer of metal or a metal alloy and a layer of polyimide, so-called single clads, may be used for printed electrical circuits. The same applies to multilayer laminates, so-called double clads or multiclads, which comprise several metal layers and/or several polyimide layers. In certain cases, however, multilayer laminates are superior to single clads. In the case of printed circuits for example, it is often necessary to make printed conductor lines which intersect one another. The high packing densities often required, i.e. overall layer thicknesses, cannot be obtained where single clads are used, but only where double clads or multiclads are used. The present invention is concerned with multilayer laminates which are eminently suitable for the production of double clads and multiclads. In the context of the invention, double clads are understood to be laminates comprising two (metallic) substrate layers, while multiclads are understood to be laminates comprising more than two (metallic) substrate layers.
Laminates containing polyimides and substrate materials are known. In this case, the polyimide layers are often bonded to the substrate materials by a conventional adhesive. For example, U.S. Pat. No. 3,900,662 describes the bonding of polyimide to metal by an acrylate-based adhesive. Use is also made of this possibility in the laminates described in U.S. Pat. No. 3,822,175.
If double clads or multiclads are produced in accordance with the above-mentioned patent specifications in which a layer of an acrylate adhesive is situated between each metal layer and each polyimide layer, the products obtained have a number of disadvantages, namely:
(a) The overall layer thickness of the clads is considerable on account of the necessary adhesive layers, whereas low overall layer thicknesses are required for multiclads.
(b) The metal (substrate material) layer is directly joined to acrylate which is inferior to the polyimide in its dimensional stability under heat. Thus, undesirable decomposition of the acrylate often occurs during preparation of the clads for printed circuits. This decomposition occurs with the acrylate layer at the high temperatures which the metal layer encounters, for example during soldering and drilling. Since the acrylate is directly joined to the metal layer, it is not adequately protected against those temperatures.
(c) Since the acrylate has poorer electrical insulating properties than the polyimide, the adhesive layer(s) between the polyimide and substrate material (metal) adversely affect(s) the dielectric properties.
It has been found that, where conventional adhesives, such as those based on acrylate, epoxide, polyamide, phenolic resin, etc. are used, the laminates in which the polyimide is bonded to the metal by an intermediate layer of one of these adhesives do not show entirely satisfactory properties which meet the stringent demands often imposed.
On account of the disadvantages of laminates comprising layers of conventional adhesives between polyimide and metal, multilayer laminates have been proposed wherein the polyimide is bonded directly to metal, i.e. without a layer of adhesive. For example, DE-OS No. 32 15 944 describes laminates in which two metal layers are bonded by an intermediate layer of polyimide. The polyimide used in this case predominantly consists of diphenyl tetracarboxylic acid and may be bonded to a metal foil by applying high temperature and pressure. In other words, the polyimide is formable. It has now been discovered that formable polyimides or polyimides which are soluble in phenolic solvents are inferior in their thermal stability to fully aromatic, no longer formable polyimides which are insoluble in phenolic solvents. In double clads which only contain these formable polyimides as an insulating layer(s) the polyimide may flow away in the process of laminating, resulting in an undesirable direct contact between the metal layers. Accordingly, clads containing only formable polyimides are inferior to products containing no longer formable polyimides as an insulating layer(s).
Because of the disadvantages associated with clads containing a layer of adhesive between metal and polyimide, single clads of a substrate material to which a no longer formable, fully aromatic polyimide which is insoluble in phenolic solvents is directly bonded have already been proposed. These single clads show excellent mechanical, thermal and electrical properties.
Starting out from these single clads, it would be desirable to produce multilayer laminates which like their single clad counterparts consist only of substrate materials and these no longer formable, fully aromatic polyimides and which would thus show the same mechanical, thermal and electrical properties. However, it has been found that two or more single clads of this type cannot be directly bonded to one another or one single clad directly bonded to a metallic substrate material. i.e. without an intermediate layer of adhesive, because it is not possible to apply another layer of substrate material or another single clad to the fully hardened polyimide layer without a coupling layer sufficient to impart a high peel strength, i.e. high adhesion between the polyimide and the additional layer. Although application of the other layer of substrate material before the polyimide has completely hardened is possible in principle and leads to an increase in peel strength, bubbles can be formed in the polyimide layer because volatile constituents such as, for example, water have to escape during its hardening and the release of these volatile constituents can be impeded by the additional layer of substrate material.